1. Technical Field
The present invention relates to an algorithm for optimizing a number of spreading codes used for channelization (channelization-code set) as well as the type of modulation and (error correction) coding scheme (MCS) used for modulating a physical channel transmitted over a radio link in a mobile telecommunications network, as well as signaling the optimized MCS and channelization-code set information and, more particularly, to extending adaptive modulation and coding in High Speed Data Packet Access (HSDPA) and Wideband Code Division Multiple Access (WCDMA).
2. Discussion of Related Art
Scheduling and adaptive modulation and coding have been considered in wireless communications in both an academic and a commercial context. For instance, in a thesis entitled “On Scheduling and Adaptive Modulation in Wireless Communications,” by Nilo Kasimiro Ericsson (June 2001), it is pointed out that adaptivity on all levels is the most important means for achieving high bandwidth efficiency in the wireless link while satisfying the required communication quality for the served applications. It is further noted that utilization of knowledge of the channel state avoids a worst-case design by adapting as conditions vary. It is shown that rather than coping with channel variations in a worse-case manner, the modulation format can be changed as the channel signal-to-noise ratio (SNR) varies. See, for instance, FIG. 5.3 on page 29 of the above-mentioned thesis. For a selected symbol error probability the modulation level can be adapted according to the variations in the channel SNR. The same subject is treated in a thesis by Peter Malm, entitled “Channel Separation, Alphabet Size, and Code Rate in Cellular Radio Systems,” dated February 1999. Maim analyzes how adjacent channel separation, channel alphabet size, and channel code rate affects the cellular system's performance. One of the conclusions of the analysis is that the spectrum efficiency of a cellular system does not improve beyond a certain point with increasing channel alphabet size. Instead, small-to-medium alphabet sizes exhibit the best performance. This is because the frequency reuse mechanism punishes large alphabets by demanding a large cluster size. As the cluster size increases faster than the throughput, the spectrum efficiency does not benefit from large-channel alphabets.
Reference may also be had to the current literature such as “Adaptive Coded Modulation for Fading Channels” by Goldsmith et al, IEEE Transactions on Communications, Vol. 46, No. 5, May 1998 and “Symbol Rate and Modulation Level-Controlled Adaptive Modulation/TDMA/TDD System for High-Bit-Rate Wireless Data Transmission” by T. Ue et al, IEEE Transactions on Vehicular Technology, Vol. 47, No. 4, November 1998 for the state of the art.
In the standards realm, the Third Generation Partnership Project (3GPP) has undertaken a study to determine the feasibility for high-speed downlink packet access (HSDPA), where techniques like adaptive modulation and coding, hybrid ARQ (HARQ) and other advanced features are discussed and evaluated with the goal to increase throughput, reduce delay and achieve high peak rates. See 3GPP TR 25.950 v4.0.0 (2001–03) “UTRA High-Speed Downlink Packet Access,” Release 4. In the foregoing 3GPP document, it is explained that in cellular communication systems, the quality of the signal received by a UE varies depending on a number of factors—the distance between the desired and interfering base stations, path loss exponent, log-normal shadowing, short-term Rayleigh fading and noise. In order to improve system capacity, peak data rate and coverage reliability, the signal transmitted to and by a particular user can be modified to account for the signal quality variation through a process commonly referred to as link adaptation. Traditionally, CDMA systems have used fast power control as the preferred method for link adaptation.
In the 3GPP study, Adaptive Modulation and Coding (AMC) have been envisioned as an alternative link adaptation method that promises to raise the overall system capacity. AMC provides the flexibility to match the modulation-coding scheme to the average channel conditions for each user. With AMC, the power of the transmitted signal is held constant over a frame interval, and the modulation and error correction coding format is changed to match the current received signal quality or channel conditions. In a system with AMC, users close to the base station (BTS) are typically assigned higher order modulation with higher code rates (e.g., 64 QAM with R=3/4 turbo codes), but the modulation-order and/or code rate will decrease as the distance from BTS increases. AMC is most effective when combined with fat-pipe scheduling techniques, such as those enabled by the Downlink Shared Channel (DSCH) of the 3GPP. On top of the benefits attributed to fat-pipe multiplexing, AMC combined with time domain scheduling offers the opportunity to take advantage of short-term variations in a UEs fading envelope so that a UE is always being served on a constructive fade.
In the spreading code domain, it has been suggested that HSDPA transmission might be able to use a fixed spreading factor and multi-code transmission. See TR 25.950 v4.0.0 (2001–03) at Chapter 6.3.1. The selection of such a fixed HSDPA spreading factor would be based on an evaluation of the impact on performance, UE complexity, and flexibility (granularity in the overall allocation of capacity for HSDPA transmission). Consideration is also recommended to what possible extent there could be any additional flexibility advantage in supporting a variable spreading factor for HSDPA as compared with the impact on complexity, etc. See 3G TS 25.213 v4.0.0 (2001–03) “Spreading and modulation (FDD)” and 3G TS 25.223 v4.0.0 (2001–03) “Spreading and Modulation (TDD)” for a general overview of spreading, including channelization, scrambling and QPSK modulation.
Although scheduling for adaptive modulation and coding has already been considered for wireless communications, there has been no concrete proposal yet advanced for a packet scheduling algorithm that optimizes the user throughput based on the selection of the number of Orthogonal Variable Spreading Factor (OVSF) codes in conjunction with the modulation and (error correcting) coding scheme (MCS) in a WCDMA network. Moreover, there has not been any recognition of the need for signaling an appropriate power level based on such an optimization.